The optical noise performance of optical amplifiers can be characterised by two main performance parameters, namely the noise figure (NF) and the multipath interference (MPI). In a highly inverted erbium doped fibre (EDF) amplifier not only is the signal amplified by stimulated emission, but also spontaneous emission is generated. This spontaneous emission can be further amplified by the amplifier resulting in amplified spontaneous emission (ASE). ASE is generated over the gain wavelength region between and underneath the data channels. At the receiver it is possible to filter out most of the ASE between the channels, but not the noise underneath the data channels. This leads to the definition of the signal spontaneous noise figure NFs-sp.
In a generic amplifier with an input stage comprising a tap coupler, an isolator and other optical components the losses in the input stage are added to the other losses in a dB for dB regime. Therefore it is important to minimize the losses in the input stage in order to optimise the noise performance of the amplifier. By dividing the EDF amplifier into imaginary sections with constant gain and close to exponential decay of pump power and inversion along a forward pumped amplifier, it can be demonstrated that the highest gain is in the first section of the amplifier. Similarly to the relationship between the input losses and the NF, high gain in the first section decreases the overall NF. In summary it can be said that, for a low NF amplifier, the input stage losses have to be minimized and the signal has to be amplified as early as possible.
Reflections in the optical signal path within the EDF amplifier are produced at different points. Passive components such as dielectric filters, optical circulators and isolators are made of bulk optic components inserted between collimating and collecting optical elements. Furthermore each of the optical surfaces produces a reflection, even if such reflection has been minimized by use of antireflection coatings. A typical return loss is ˜55 dB, although pump laser diodes can have a much higher return loss of the order of ˜20 dB and fibre Bragg gratings can have a return loss down to 1 dB. Additionally Rayleigh scattering in optical fibres provides a reflection mechanism in the fibre itself, although compared to discrete reflections the Rayleigh scattering can usually be ignored.
Double reflections of the signal in the amplifier (higher order reflections being ignored) or multipath interference (MPI) act as another source of noise. In an EDF amplifier this noise cannot be measured in the optical domain by contrast with the signal spontaneous noise figure. Furthermore, in the case where there is a highly inverted EDF between two reflection points in the amplifier, the MPI can reach levels that will deteriorate the amplifier performance significantly.
U.S. Pat. No. 6,377,394 discloses a control arrangement for controlling pump currents supplied to a number of pump stages of an EDF amplifier in order to achieve a substantially constant gain in spite of the various factors tending to give rise to gain modulation. However, at low levels of amplification, current is supplied only to the first pump stage, and no current is supplied to the or each further pump stage. As a result losses are associated with the further pump stages which are in turn compensated for by an increase in the power supplied to the first pump stage, thus providing a high level of MPI and deteriorating the performance of the amplifier.
It is an object of the invention to provide an optical amplifier having a low noise characteristic which is maintained over time and under various conditions.